So I am guessing you are using Windows Vista (any version) 64-bit and you just installed Quicktime for windows to discover a black bar instead of a progress bar with a mute button in Firefox when watching trailers or audio files… well people I had the exact same problem and here’s the fix:
Either go to your desktop icon for Firefox or go to the main Firefox folder which should be located in Computer/Local DIsk (C:)/Program Files (x86)/Mozilla Firefox
1. Right click on the browser icon
2. Select properties
3. Go to the compatibility tab
4. Check the box for “Run this program in compatibility mode for:”
5. Make sure it says “Windows XP (Service Pack 2)” in the drop down list.
6. Press “OK” at the bottom
Restart Firefox if it was launched while you did this.